The work shows so-called “blocking patterns”, where
hot or wet weather remains stuck over a region for weeks causing heatwaves or
floods, have more than doubled in summers over the last decade. The new study
may also demonstrate a link between the UK’s recent flood-drenched winter and climate change.

Climate scientists in Germany noticed that since 2000
there have been an “exceptional number of summer weather extremes, some causing
massive damage to society”. So they examined the huge meanders in the
high-level jet stream winds that dominate the weather at mid-latitudes, by
analysing 35 years of wind data amassed from satellites, ships, weather
stations and meteorological balloons. They found that blocking patterns, which
occur when these meanders slow down, have happened far more frequently.

“Since 2000, we have seen a cluster of these events.
When these high-altitude waves become quasi-stationary, then we see more
extreme weather at the surface,” said Dr Dim Coumou, at the Potsdam Institute
for Climate Impact Research. “It is especially noticeable for heat extremes.”
The intense heatwaves in Russia in 2010, which saw 50,000 people die and the wheat harvest hit hard, and in
western Europe in 2003, which saw 30,000 deaths, were both the result of
blocking patterns. The Intergovernmental Panel on Climate Change concluded in
2011 that extreme weather would become more
common as global warming heats the planet, causing
both heatwaves and increasingly severe rain storms.

The rise in blocking patterns correlates closely with
the extra heating being delivered to the Arctic by climate change, according to
the research which is published in the journal Proceedings of the
National Academies of Science
(PNAS). Coumou and his colleagues argue there are good physical reasons to
think there is a causal link, because the jet streams are driven by the
difference in temperature between the poles and the equator. As the Arctic is
warming more quickly than lower latitudes, that temperature difference is
declining, providing less energy for the jet stream and its meanders, which are
called Rossby waves.

Prof Ted Shepherd, a climate scientist at the
University of Reading, UK, but not involved in the work, said the link between
blocking patterns and extreme weather was very well established. He added that
the increasing frequency shown in the new work indicated climate change could
bring rapid and dramatic changes to weather, on top of a gradual heating of the
planet. “Circulation changes can have much more non-linear effects. They may do
nothing for a while, then there might be some kind of regime change.”

Shepherd said linking the rise in blocking events to
Arctic warming remained “a bit speculative” at this stage, in particular
because the difference between temperatures at the poles and equator is most
pronounced in winter, not summer. But he noted that the succession of storms
that caused England’s wettest winter in 250
years was a “very good example” of blocking patterns
causing extreme weather during the coldest season. “The jet stream was stuck in
one position for a long period, so a whole series of storms passed over
England,” he said.

Coumou acknowledges his study shows a correlation –
not causation – between more frequent summer blocking patterns and Arctic
warming. “To show causality, computer modelling studies are needed, but it is
questionable how well current climate models can capture these effects,” he
said.

Prof Tim Palmer, at the University of Oxford, wrote in
a PNAS article in 2013 that understanding changes to blocking patterns may well be the key to
understanding changes in extreme weather, and therefore to understanding the
worst impacts of climate change on society. But he said climate models might
have to run down to scales of 1km to do so. “Currently, national climate
institutes do not have the high-performance computing capability to simulate
climate with 20km resolution, let alone 1km,” he wrote. “[I] look forward to
the day when governments make the same investment in climate prediction as they
have made in finding the Higgs boson.”